Cholesteric liquid crystal materials are known and disclosed in U.S. Pat. Nos. 5,437,811; 5,695,682; 5,453,863; and 5,691,795, all of which are assigned to the assignee of the present invention and which are incorporated herein by reference. The primary advantage of the bistable cholesteric liquid crystal materials disclosed in these patents is that they can be driven to a desired texture with application of a voltage and remain in that texture after removal of the applied voltage. As seen in FIG. 1, bistable cholesteric liquid crystal materials are known to exhibit at least four states or textures: homeotropic, focal conic, transient planar, and planar. Both the homeotropic and transient planar textures are considered transitory and do not remain after removal of an electric field. These transitory textures are employed to facilitate the transformation of the cholesteric liquid crystal material into either a weakly light scattering, transmissive focal conic texture or a reflective planar texture.
The next step in the development of bistable cholesteric liquid crystal devices was focused on how to drive the cholesteric liquid crystal material quickly between the focal conic and planar textures. This development is necessitated by the desire to provide efficient operation of the device, with as fast as possible update rates. Such driving schemes are found in U.S. Pat. No. 5,748,277, and in U.S. patent application Ser. No. 08/852,319, both of which are owned by the assignee of the present invention and which are incorporated herein by reference. Initially, a three phase dynamic drive scheme, as shown in FIGS. 2A and 2B, was employed to control the appearance of the cholesteric device. As is discussed in the above patents, the liquid crystal material is disposed between two substrates, one of which has a plurality of row electrodes and the other which has a plurality of column electrodes orthogonal to the row electrodes. Application of voltage waveforms to the electrodes is multiplexed or applied in a predetermined sequence. Hence, these displays are sometimes referred to as multiplexed displays. Those skilled in the art will appreciate that multiplexed displays are not limited to "row and column" electrode patterns. Segmented liquid crystal displays, such as clock faces and calculator displays, may also be multiplexed. In either type of display the term "common electrode" may be used to refer unto a row electrode, and the term "segment electrode" may be used to refer to a column electrode.
The liquid crystal material in between the intersecting electrodes form a pixel. As shown in FIGS. 2A and 2B, the appearance of each pixel is controlled by a pixel voltage waveform which comprises a sequence of three RMS voltages: V.sub.preparation, V.sub.select/non-select, and V.sub.evolve. V.sub.preparation or V.sub.p drives the cholesteric liquid crystal material into the homeotropic texture regardless of its initial texture. Application of V.sub.select/non-select or V.sub.s/ns determines if the homeotropic texture relaxes into the planar (V.sub.select) or the focal conic texture (V.sub.non-select). The evolution voltage or V.sub.e serves two functions. First, it permits the focal conic texture to evolve from the transient planar texture that results from applying V.sub.non-select. The evolution voltage also restores and maintains the homeotropic texture after V.sub.select is applied allowing relaxation to the planar texture which occurs when V.sub.evolve is removed. It has been determined that display update speed can be increased by applying the voltages V.sub.preparation and V.sub.evolve across many rows simultaneously. Once V.sub.evolve is removed from the last addressed row, all power is removed from the display and the desired indicia appears on the display.
Implementation of such a drive scheme has proven to be quite costly. In particular, previous displays required 50-60V (RMS) to drive the cholesteric liquid crystal material into the homeotropic texture from which it relaxes into the reflective planar texture. Since the use of cholesteric liquid crystal materials in displays is relatively new, there are no commercially available electronic driving circuits uniquely designed to apply the necessary voltage waveforms to a display.
One option that was initially investigated was to employ a multiplexed super twisted nematic (STN) display driver. STN displays are addressed constantly so that each pixel always has an applied voltage across it that is the combination of waveforms being applied to the appropriate intersecting electrodes. The "state" or texture of a particular pixel (on or off, light or dark) depends on the average voltage across the pixel during a single scan or update of the display. The difference between the average voltages of these two pixels states is small, on the order of about 0.1 volt. This difference is generated entirely by the choice of voltage, either high or low, applied to the pixel while it is selected for update. The number of DC voltage levels required to drive a STN display is relatively small. Four voltage levels are required for each common/row and segment/column waveform and typically, two of these voltage levels are common to both. Accordingly, only six distinct DC voltage levels, which are separate from the logic voltage inputs, are required to address an STN display. STN driver chips also include a data input called the frame line that selects between two fixed pairs of display voltage inputs for all the outputs on a chip. For example, if the display voltage inputs are labeled V.sub.1, V.sub.2, V.sub.3, and V.sub.4, the frame line can select between either the pair V.sub.1 and V.sub.2 or the pair V.sub.3 and V.sub.4. No other selections are possible. Of course other label designations could be used for the voltage inputs. Each STN driver chip also includes a shift register containing one data bit per chip output. Each bit selects one of the two display voltage inputs selected by the frame line. Accordingly, each bit can select between V.sub.1 and V.sub.2 or between V.sub.3 and V.sub.4. Once again, no other selections are possible. The voltages applied to the display voltage inputs must obey strict rules. At a minimum, the rule (V.sub.4.gtoreq.V.sub.3.gtoreq.V.sub.2.gtoreq.V.sub.1) must be obeyed. Moreover, it is typical to require two of the four display voltage inputs (V.sub.3 and V.sub.4) to be set very near to the chip's upper supply voltage while the other two display voltage inputs (V.sub.1 and V.sub.2) are set very near the chip's lower supply voltage. These requirements are intended to ensure proper chip operation and are primarily a function of the chip design.
Although it was desired to employ the STN driver chips to drive the cholesteric liquid crystal display because of their relatively low cost (about 2 cents per output), it was readily apparent that the drive scheme requirements of cholesteric liquid crystal displays were significantly more severe than super twisted nematic displays. The state of a STN pixel depends only on the average voltage across the pixel during a single update of the display and not on the specific sequence of voltages applied to the pixel. While cholesteric liquid crystal displays respond to the average voltages applied to them, the state of a pixel depends on the sequence of RMS/average voltages applied during an update. As noted previously, the dynamic address scheme requires the proper application of RMS voltages V.sub.p, V.sub.s/ns, and V.sub.e in order to select between the two stable cholesteric liquid crystal textures. The only known way to address cholesteric liquid crystal displays with the dynamic drive scheme was to employ high voltage analog switches to generate the necessary row waveforms.
A first attempt at employing STN drivers resulted in providing half of the signals needed to drive a cholesteric liquid crystal display. In this approach, the STN driver chips were employed to generate the column waveforms and high voltage analog switches were employed to generate the row waveforms. The row waveforms were AC waveforms, and the necessary RMS voltages were generated entirely by these row waveforms. The column waveforms supplied by the segment/column drivers were of small amplitude and amounted to inconsequential noise on all rows except the row being addressed. On the row being addressed, the row waveform voltage levels were comparable to the column waveform (data) voltage levels. As such, the proper select and non-select voltages could be generated by changing the phase of the column waveforms.
The fundamental characteristic of STN driver chips that led to this hybrid mixture of driver chips and analog switches is that STN driver chips are "unipolar," that is, the output voltages can range, for example, from 0-40 volts, as opposed to "bipolar" wherein the output voltages would range from -40V to +40V. It was not thought possible to generate the necessary RMS voltages given the limited voltage range of STN drivers, typically no more than 40 volts, versus the 200 volts required for the analog switches used in the initial embodiment. In particular, the initial embodiment was designed so that one high voltage analog switch chip was needed to drive every two rows. At eight switches per chip, four separate analog switches controlled each row. Accordingly, a four inch by four inch display used in the initial embodiment had 320 rows, so 160 of the high voltage analog switch chips were required. This forced the cost of the drivers alone to over $3,000.00. Although the drivers in association with the other circuitry were effective in driving the display, it was quite cost prohibitive. Moreover, scaling up to a page-size display at a reasonable resolution (133 DPI) was clearly out of the question in attempting to develop a commercially cost-effective cholesteric display.